fix: avoid entry author deadlocks

[mvanhorn]

Description

Change Entry::_saveAuthors() to use the existing Db::deleteIfExists() helper for entries_authors, which performs the delete only when matching rows exist and was designed to avoid deadlocks caused by deleting absent rows. Keep the existing author lookup and batch insert behavior intact so author replacement on established entries and initial author persistence continue to work across supported databases. Add a focused regression in EntriesTest that observes the database commands for an initial authored-entry save versus an author update: the initial save must not execute a delete for its absent author rows, while the update must still remove the existing rows and persist the replacement author.

Concurrent entry creation can intermittently deadlock on MySQL or MariaDB while Entry::_saveAuthors() writes to entries_authors. The thread includes reproductions from queue jobs, load-balanced installations, Craft Cloud, and a core-only control-panel flow, with InnoDB diagnostics showing distinct new entry IDs contending on the primary-index supremum. The save path currently issues an unconditional delete for an entry ID before inserting its authors, even when a newly-created entry cannot have author rows yet. That empty-range delete can take a gap lock that conflicts with another transaction following the same delete-then-insert sequence.

Fixes #15768

Related issues

Not applicable to this change.

[brandonkelly]

Thanks for the PR!

You’ve got a couple PhpStan issues to address in testSaveAuthorsAvoidsDeletingMissingRows() though.